Today, about two-thirds of U.S. adults are overweight or obese, according to the Centers for Disease Control and Prevention. Obesity is harmful to physical health as well as an established risk factor for a number of potentially life-threatening diseases such as atherosclerosis, hypertension, diabetes, stroke, pulmonary embolism, and cancer. Moreover, obesity can wreak havoc on an individual's mental health and can affect a person's ability to interact socially with others.
Accompanying the devastating medical consequences of this problem is the severe financial burden placed on the health care system in the United States. The estimated economic cost of obesity and its associated illnesses from medical expenses and loss of income are reported to be in excess of $68 billion per year. Because of the impact of obesity on individuals and society, much effort has been expended to find ways to treat obesity, but little success has been achieved in the long-term treatment and/or prevention of obesity.
Pro-opiomelanocortin (POMC) derived peptides are known to affect food intake. Several lines of evidence support the notion that the G-protein coupled receptors (GPCRs) of the melanocortin receptor (MCR) family, several of which are expressed in the brain, are the targets of POMC derived peptides involved in the control of food intake and metabolism. Physiologically, the melanocortin system has been recognized to participate in the regulation of feeding behavior, obesity, and energy homeostasis in rodents as well as humans (Huszar, D. et al., “Targeted disruption of the melanocortin-4 receptor results in obesity in mice,” Cell, 88:131-141 (1997); Fan, W. et al., “Role of melanocortinergic neurons in feeding and the agouti obesity syndrome,” Nature, 385:165-168 (1997); Krude, H. et al., “Severe early-onset obesity, adrenal insufficiency and red hair pigmentation caused by POMC mutations in humans,” Nat. Genet., 19(2):155-157 (1998); Vaisse, C. et al., “Melanocortin-4 receptor mutations are a frequent and heterogeneous cause of morbid obesity,” J. Clin. Invest., 106(2):253-262 (2000); and Farooqi, I. S. et al., “Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene,” N. Engl. J. Med., 348(12):1085-1095 (2003)).
Five distinct MCRs have thus far been identified, and these are expressed in different tissues. MC1R was initially characterized by dominant gain of function mutations at the Extension locus, affecting coat color by controlling phaeomelanin to eumelanin conversion through control of tyrosinase. MC1R is mainly expressed in melanocytes. MC2R is expressed in the adrenal gland and represents the ACTH receptor. MC3R is expressed in the brain, gut, and placenta and may be involved in the control of food intake and thermogenesis. MC4R is uniquely expressed in the brain, and laboratory observations suggest that it is also involved in the control of food intake. See Kask A, et al., “Selective antagonist for the melanocortin-4 receptor (HS014) increases food intake in free-feeding rats,” Biochem. Biophys. Res. Commun., 245:90-93 (1998)). MC5R is expressed in many tissues, including white fat, placenta and exocrine glands. MC5R knockout mice reveal reduced sebaceous gland lipid production (Chen et al., “Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides,” Cell, 91:789-798 (1997)).
The melanotropin peptides, α-, β- and γ-melanocyte stimulating hormone (MSH) and adrenocorticotropin hormone (ACTH), are the endogenous agonists of the melanocortin receptors that are derived by posttranslational processing of the POMC precursor hormone (see, for example, Eipper, B. A. and Mains, R. E. (1980) Endocrin. Rev., 1:1-26; and Smith, A. I. and Funder, J. W. (1988) Endocrin. Rev., 9:159-179). The endogenous melanocortin receptor antagonist, Agouti-related protein ((AGRP) is a signaling molecule involved in weight homeostasis that, when overexpressed, has been shown to cause adult onset obesity and diabetes.
AGRP is a competitive antagonist of melanocortin agonist ligands at the centrally expressed melanocortin-3 (MC3R) and melanocortin-4 (MC4R) receptors (Ollmann, M. M. et al. (1997) Science, 278:135-138). In vitro pharmacological studies have demonstrated that AGRP also possesses inverse agonist activity at the MC4R (Chai, B. X. et al., “Inverse agonist activity of agouti and agouti-related protein,” Peptides, 24(4):603-609 (2003); Haskell-Luevano, C. and Monck, E. K., “Agouti-related protein functions as an inverse agonist at a constitutively active brain melanocortin-4 receptor,” Regulatory Peptides, 99:1-7 (2001); and Nijenhuis, W. A. et al., “AgRP(83-132) acts as an inverse agonist on the human-melanocortin-4 receptor,” Mol. Endocrinol., 15(1): 164-171 (2001)).
All endogenous melanocortin agonists contain the putative amino acid sequence (His)/Phe-Arg-Trp, postulated to be important for melanocortin receptor molecular recognition and stimulation. Further extrapolation of the homology between the antagonist Arg-Phe-Phe motif and the endogenous melanocortin agonist conserved residues Phe-Arg-Trp, implies that the antagonist residues may be mimicking the agonist Phe-Arg-Trp interactions with the melanocortin receptors, as supported by Tota, M. R., et al., “Molecular interaction of Agouti protein and Agouti-related protein with human melanocortin receptors,” Biochemistry, 38:897-904 (1999) and Haskell-Luevano, C., et al., “The agouti-related protein decapeptide (Yc[CRFFNAFC]Y) possesses agonist activity at the murine melanocortin-1 receptor,” Peptides, 21:683-689 (2000).
Previous structure-activity studies of the agouti peptide identified the importance of the three amino acid motif Arg-Phe-Phe that is conserved in both agouti and AGRP (see, for example, Kiefer, L. et al., “Mutations in the carboxyl terminus of the agouti protein decrease agouti inhibition of ligand binding to the melanocortin receptors,” Biochemistry, 36:2084-90 (1997)). These studies suggest that the conserved Arg-Phe-Phe motif found in both agouti and AGRP may be important for the antagonistic and molecular recognition properties of these two molecules at the melanocortin receptors.
Tremendous efforts are being performed in attempts to identify genetic disorders resulting in human obesity and to identify drugs that can correct for the dysfunction of these protein polymorphisms. It has been clearly demonstrated that the MC4R, POMC derived endogenous agonists, and endogenous MC4R antagonist Agouti-related protein (AGRP), when modified, result in an obese phenotype in both mice and humans (see, for example, Ollmann, M. M. et al. (1997) Science, 278:135-138; Huszar, D. et al. (1997) Cell, 88:131-141; Hinney, A. et al. (1999) J. Clin. Endocrinol. Metab., 84(4):1483-1486; Yaswen, L. et al. (1999) Nat. Med., 5(9):1066-1070; Yeo, G. S. et al. (1998) Nat. Genet., 20(2):111-112; Vaisse, C. et al. (1998) Nat. Genet., 20(2):113-114; and Hinney, A. et al. (1998) J. Clin. Endocrinol. Metab., 83(10):3737-3741). For example, it has been demonstrated that targeted disruption of the MC4R gene in mice results in hyperphagia and obesity (Huszar, D. et al., “Targeted disruption of the melanocortin-4 receptor results in obesity in mice,” Cell, 88:131-141 (1997)).
MC4R has a role in regulating energy homeostasis and obesity. In humans, more than 50 different single nucleotide polymorphisms (SNPs) of the MC4R gene have been discovered. Up to a remarkable 6% of morbidly obese adults and children studied possess single heterozygous nucleotide polymorphisms (SNPs) of the MC4R. Unfortunately, to date, very little data is available other than hypotheses regarding the structure-function relationship of MC4R to the genetic predisposition to obesity in humans. In view of the need to better understand the genetics behind obesity and its relationship with MCRs, new agents, methods, and compositions for treating or preventing obesity resulting from MC4R polymorphisms need to be identified and developed.